The present invention relates to a sweep signal generating device for use with, for example, a spectrum analyzer, a frequency characteristic monitor or the like, which is adapted to sweep a frequency by providing a corresponding sweep signal.
This kind of sweep signal generating device is used, for example, in the following manner. A frequency-swept signal is supplied to an input of an object to be measured, and an output signal from the object is applied to a vertical deflection circuit of a cathode ray tube display while applying a sweep signal corresponding to the frequency-swept signal to a horizontal deflection circuit of the cathode ray tube display. Thus the frequency characteristic of the output signal is displayed on the screen of the cathode ray tube display. In conventional sweep signal generating devices, the relationship of correspondence between a position on the abscissa on the screen, that is, in the direction of horizontal sweep, and the sweep frequency is not accurately defined. It is general practice to display frequency marks on the screen so as to ensure accurate reading of the frequency of the waveform displayed on the screen. But this involves a relatively complicated arrangement and it is difficult to make a predetermined frequency correspond exactly to a predetermined position on the screen. The U.S. Pat. No. 4,130,808 patented on Dec. 19, 1978 has proposed such a sweep signal generating device. In this prior art, the output from a variable frequency oscillator and the output from a reference signal oscillator are phase-compared with each other and, by the phase-compared output, the oscillation frequency of the variable frequency oscillator is controlled to coincide with a reference frequency thus providing a phase locked loop. After the phase locked loop is established, an error signal in the phase comparison is retained and, at the same time, the phase locked loop is opened and a sweep signal, that is, a ramp voltage is generated. The oscillation frequency of the variable frequency oscillator is varied in a certain direction, that is, upwardly or downwardly of the reference frequency, and when the ramp voltage reaches a predetermined value, the sweep is stopped.
According to this sweep signal generating device, the sweep start point coincides with the reference frequency signal with high accuracy; accordingly, in the case of providing a display, for example, on a cathode ray tube display, the position of one end of the display which is the sweep start point indicates correctly the reference signal frequency. However, the frequency at the sweep end point, that is, at the other end of the display is dependent on the linearity of the frequency characteristic of the variable frequency oscillator with respect to a control signal therefor, and if the linearity is poor, the frequency at the sweep end point is appreciably scattered; hence, the frequency cannot accurately be known. Further, in the case of using, as the reference frequency signal, the output from a reference frequency generator of a fixed frequency, for example, a crystal oscillator, the frequency at the sweep start point is fixed specifically to the reference frequency and the sweep start frequency cannot be set as desired. Moreover, if a synthesizer, for example, is employed to provide the output as the reference frequency signal, then the manufacturing cost of the sweep signal generating device becomes relatively high.